# [ODE] help for check the direction of the fdir1 in the wheel simulation

Jon Watte (ODE) hplus-ode at mindcontrol.org
Fri Nov 23 11:10:11 MST 2007

```Because you don't want slip in the forward direction; you only want to
simulate slip in the side direction. The rolling physics eat up the slip
in the forward direction, unless you lock the brakes. Also, there is no
"sideways" velocity in the forward direction, so thus no force dependent
slip!

Cheers,

/ h+

> From the ODE's  user guide, it says "set friction direction 1 in the
> direction that the tire is rolling in, and set the FDS slip coefﬁcient
> in friction direction 2 to k ∗v".
>
> So if the wheel is rolling, the fdir1 will be the direction of the
> linear velocity, and the fdir2 will be the side direction of the
> wheel. And also we can say the fdir2 is the direction of  rolling axis.
>
> So, why we need set the slip coefficient on the side direction but not
> along the direction of the linear velocity?
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